So, it sounds like you've got some great experience.
1) Many SMPs require a minimum of a 3.0 GPA to apply/be considered. There's a reason: because med schools don't like to consider undergraduate GPAs below 3.0. With an undergrad GPA of 3.0 and a graduate GPA of, say, a 3.8, you have a shot. But with an undergrad of 2.75, at an allopathic school, anyway, it's a long shot. If you are interested in DO, talk to an advisor--they might have some better news.
2) If you're serious about getting your GPA up, I would pull up the undergrad gpa. However, if you're serious about it, you have two options: work full-time and take one class at a time (at two classes you might burn out), or devote yourself to school full-time, raise those grades, and eat the costs in loans.
I think you can definitely do DO. If you want to pursue the MD, I think you can do that do, but I think you should talk to an adviser about it: make a five-year plan for work, school, MCAT, and personal time and goals (outside of medicine). The MD is going to take you more time to get to. If you can go back to school full-time for two or three years, make that your goal. Then, if you still want to become an even more competitive applicant, do an SMP. With your background, an above-3.0 undergrad GPA, a master's degree with hopefully a good gpa, and some more maturity on you by then (24 is great, but 28 makes you even more appealing to med schools), with a good MCAT, you'll be in. You'll be up to your ears in loans, but you can defer them all till you're done, and you might consider a military scholarship or National Health Service Corps.
Best of luck!